1. Field of the Invention
The present invention relates to image forming apparatuses such as a copying machine and a printer in which an electrophotographic system, and an electrostatic recording system are used.
2. Related Background Art
In a conventional image forming apparatus, after applying toner as a developer to a latent image bearer bearing a latent image in accordance with image information to form the latent image into a toner image, the toner image is transferred to a transfer material as a recording medium. An image forming apparatus provided with a separating charger as separation charging means for forming an electric field between the transfer material with the toner image transferred thereon and the latent image bearer to separate the transfer material from the latent image bearer is known, and placed for practical use.
Moreover, in the image forming apparatus, it is proposed that an electric current for the separating charger to form the electric field (hereinafter referred to as the separation current) be appropriately adjusted in accordance with the total amount of image information to the transfer material. For example, as an extreme example, in a solid white image (image information amount of zero), since no toner particle is between the latent image bearer and the transfer material, the electrostatic adsorbability of the transfer material to the latent image bearer becomes very large, and poor separability tends to occur, which requires a strong electricity eliminating effect and a large separation current. On the other hand, in a solid black image (the maximum image information amount), when the electricity of the transfer material is strongly eliminated, the toner once transferred onto the transfer material is reverse-transferred onto the latent image bearer, that is, a so-called re-transfer phenomenon occurs. Therefore, the separation current needs to be set to a not very large value. Therefore, it is useful to adjust the separation current in accordance with the total amount of image information. Therefore, in the conventional apparatus, the separation current is controlled in accordance with the image density of the original to be read as described in Japanese Patent Application Laid-Open No. 62-159165, or the image ratio of the entire original is calculated to control the separation current in accordance with the ratio as described in Japanese Patent Application Laid-Open No. 10-78705.
However, when the separation current calculated based on the image ratio of the entire original is supplied through the entire transfer material, the separation current does not necessarily take an optimum value, unnecessarily much separation current is supplied, and re-transfer is caused in latter half portion of the transfer material in the conveying direction.
Moreover, when the image ratio is calculated from the read original to determine the separation current, and when enlargement, reduction, rotating negative/positive reversing, or another processing is performed, the image ratio of the original is different from the image ratio of the image transferred onto the transfer material. Therefore, too much or too less separation current is supplied to the transfer material, and the jamming of the transfer material by re-transfer or poor separability is caused in some cases.